DGR made a number of similar drawings of Jane Morris in 1870: see
for example, the series of drawings DGR made of her between April and the
end of 1870, the earliest of which seems to have been
the drawing done
on 30 April 1870 (in the Ashmolean), which is very close
to the present drawing in general conception.
The drawing could possibly belong to the spring of 1870, when DGR and Mrs. Morris
were invited by Barbara Bodichon to stay at her home in Scalands. But it could also
be later in the year, as were many of the drawings he made of her in 1870.